Electrolyte for electrolytic capacitor and electrolytic capacitor having the same

ABSTRACT

The present invention provides an electrolytic capacitor having excellent high temperature life characteristics and excellent low temperature properties, an aluminum electrolytic capacitor having excellent high temperature life characteristics and moisture resistance, a low dielectric loss and good low temperature properties, and an electrolyte therefor. A first electrolyte comprises a mixed solvent of sulfolane and at least one selected from 3-methyl sulfolane and 2,4-dimethyl sulfolane, a second electrolyte in which a quaternized imidazolinium salt or a quaternized pyrimidinium salt is dissolved in a mixed solvent containing sulfolane and γ-butyrolactone, and a third electrolyte comprises a mixed solvent containing γ-butyrolactone and at least two selected from sulfolane, 3-methyl sulfolane and 2,4-dimethyl sulfolane and a quaternized imidazolinium salt or a quaternized pyrimidinium. An aluminum electrolytic capacitor constructed by winding, via a separator, an anodic electrode foil provided with an anode leading means and a cathodic electrode foil provided with a cathode leading means made of aluminum to thereby form a capacitor device, and then impregnating said capacitor device with each of the above-specified electrolytes.

This is a divisional of application Ser. No. 09/213,435 filed Dec. 17,1998 now U.S. Pat. No. 6,166,899, the disclosure of which isincorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to an electrolyte for an electrolytic capacitor.More specifically, it relates to an electrolyte for an electrolyticcapacitor having excellent high temperature life characteristics, and anelectrolytic capacitor comprising the same. More particularly, itrelates to an aluminum electrolytic capacitor having excellent hightemperature life characteristics and moisture resistance.

BACKGROUND OF THE INVENTION

The general method for producing an electrolytic capacitor compriseswinding, via an separator made of Manila paper, etc., an anodicelectrode foil (A), which has been obtained by chemically orelectrochemically etching a valve metal foil (for example, a band madeof highly pure aluminum) to enlarge the foil surface and subjecting thisfoil to a anodizing treatment in an electrolyte such as an aqueousammonium borate solution to thereby form an oxide coating layer on itssurface, and a cathodic electrode foil (B) made of a highlypure foilsubjected to etching treatment only. Next, the obtained capacitor deviceis impregnated with an electrolyte for driving electrolytic capacitorsand putting in a bottomed outer case. The outer case is equipped at theopening with a sealer made of an elastic rubber and sealed by drawing.

The anodic electrode foil and the cathodic electrode foil are eachconnected to a lead wire by stitching, ultrasonic welding, etc. so as tolead the electrode. The lead wire employed as each electrode leadingmeans is composed of a round bar, a connecting member being in contactwith the electrode foil and an outer connecting member made of asolderable metal which has been fixed at the tip of the round bar bywelding, etc.

There are various electrolytes, with which capacitor devices are to beimpregnated for driving electrolytic capacitors, depending on theperformance of the electrolytic capacitor employed. Among them, adipicacid solutions in ethyleneglycol are known as electrolytes which aresuitable for low voltage and have excellent high temperature lifecharacteristics.

FIGS. 1 and 2 show general structures of aluminum electrolyticcapacitors. An aluminum foil of a high purity is chemically orelectrochemically etched to thereby enlarge the aluminum foil surface.This aluminum foil is subjected to a anodizing treatment in anelectrolyte such as an aqueous ammonium borate solution to give ananodic electrode foil (2) having an oxide coating layer formed on itssurface. This anodic electrode foil (2) and a cathodic electrode foil(3) made of an aluminum foil of a high purity having been etched alone,are wound via a separator (11) made of Manila paper, etc. to give acapacitor device (1) as shown in FIG. 2. As FIG. 1 shows, this capacitordevice (1) is impregnated with an electrolyte for driving electrolyticcapacitors and then put into a bottomed outer case (10) made of aluminumetc., which is then sealed by drawing.

As FIG. 2 shows, the anodic electrode foil (2) and the cathodicelectrode foil (3) are provided respectively with lead wires (4) and(5), which are electrode-leadingmeans, by stitching, welding, etc. Thelead wires (4) and (5) each employed as an electrode leading means iscomposed of a round bar (6) made of aluminum, a connecting member (7)being in contact with the electrode foil (2) or (3) and an outerconnecting member (8) made of a solderable metal which has been fixed atthe tip of the round bar (6) by welding, etc.

There are various electrolytes, with which the capacitor device (1) isimpregnated for driving aluminum electrolytic capacitors, depending onthe performance of the aluminum electrolytic capacitor employed. Amongthem, a solution comprising a quaternary ammonium salt dissolved inγ-butyrolactone is known as an electrolyte having a high electricconductance. In recent years, there are further reported electrolyteswherein γ-butyrolactone is employed as the main solvent and saltscomposed of quaternized cyclic amidin compounds (imidazolinium cationand imidazolium cation) as the cationic component and acid conjugatedbases as the anionic component are dissolved therein as the solute(JP-A-8-321440 and JP-A-8-321441; The term “JP-A” as used herein meansan “unexamined published Japanese patent application”).

With the recent tendency toward improved automobile functions, it hasbeen more and more required in the field of vehicle equipments to useelectronic parts in engine rooms operated at high temperatures. However,electrolytic capacitors with the use of the above-mentioned electrolytescannot withstand these high temperatures. Regarding low temperaturecharacteristics, furthermore, these electrolytic capacitors canwithstand a temperature of −25° C. at lowest but vehicle equipmentsshould generally withstand low temperatures of about −40° C. Namely, noelectrolyte for electrolytic capacitors usable in this field has beenprovided in practice so far.

Although sulfolane has been known as a high-boiling solvent capable ofimparting excellent high temperature life characteristics (JP-A-1-124210and JP-A-8-31699), it fails to establish the desired characteristics asdescribed above. Thus, sulfolane cannot be employed in vehicleequipments which should have excellent high temperature lifecharacteristics and low temperature properties simultaneously.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide anelectrolytic capacitor having excellent high temperature lifecharacteristics and excellent low temperature properties and anelectrolyte to be used in these electrolytic capacitors.

Another object of the present invention is to provide an aluminumelectrolytic capacitor having excellent high temperature lifecharacteristics and moisture resistance, a low dielectric loss and goodlow temperature properties.

The above objects are achieved mainly by the following constitutions:

(1) A first electrolyte for electrolytic capacitors comprises a mixedsolvent of sulfolane and at least one selected from the group consistingof 3-methyl sulfolane and 2,4-dimethyl sulfolane as a solvent;

the first electrolyte for electrolytic capacitors, wherein the contentof sulfolane is from 20 to 70% by weight based on the mixed solvent;

the first electrolyte for electrolytic capacitors, which comprises aquaternized cyclic amidinium salt as a solute; and

an electrolytic capacitor, which comprises the first electrolyte,

(2) An aluminum electrolytic capacitor constructed by winding, via aseparator, an anodic electrode foil provided with an anode leading meansand a cathodic electrode foil provided with a cathode leading means madeof aluminum to thereby form a capacitor device, and then impregnatingsaid capacitor device with a second electrolyte in which a quaternizedimidazolinium salt or a quaternized pyrimidinium salt is dissolved in amixed solvent containing sulfolane and γ-butyrolactone;

the aluminum electrolytic capacitor, wherein the second electrolytecontains γ-butyrolactone in an amount of from 20 to 60% by weight basedon the mixed solvent;

the aluminum electrolytic capacitor, wherein the cathodic electrode foilis an aluminum foil having provided on the whole or part of a surfacethereof a coating made of a metal nitride selected from the groupconsisting of titanium nitride, zirconium nitride, tantalum nitride andniobium nitride or a metal selected from the group consisting oftitanium, zirconium, tantalum and niobium; and

the aluminum electrolytic capacitor, wherein an aluminum oxide layer isformed by anodic oxidation on the whole or a part of the surface of thecathode leading means, and

(3) a third electrolyte for electrolytic capacitors which comprises amixed solvent containing γ-butyrolactone and at least two selected fromthe group consisting of sulfolane, 3-methyl sulfolane and 2,4-dimethylsulfolane as a solvent and a quaternized imidazolinium salt or aquaternized pyrimidinium as a solute; and an electrolytic capacitorwhich comprises the third electrolyte.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an internal sectional view showing the structure of thealuminum electrolytic capacitor.

FIG. 2 is a perspective view showing the structure of the capacitordevice.

DETAILED DESCRIPTION OF THE INVENTION

The aluminum electrolytic capacitor of the present invention has thesame structure as that of the conventional ones, as shown in FIGS. 1 and2. A capacitor device (1) is formed by winding an anodic electrode foil(2) and a cathodic electrode foil (3) via a separator (11). As FIG. 2shows, the anodic electrode foil (2) and the cathodic electrode foil (3)are connected respectively to a lead wire (4) for leading the anode andanother lead wire (5) for leading the cathode. These lead wires (4) and(5) are each composed of a connecting member (7) being in contact withthe electrode foil, a round bar (6) having been molded integrally withthe connecting member (7), and an outer connecting member (8) havingbeen fixed at the tip of the round bar (6). The connecting member (7)and the round bar (6) are each made of highly pure aluminum while theouter connecting member (8) is made of a solder-plated, copper-platedsteel wire. These lead wires (4) and (5) are connected respectively tothe electrode foils (2) and (3) at the connecting member (7) bystitching, ultrasonic welding, etc.

As the anodic electrode foil (2), use is made of one formed bychemically or electrochemically etching an aluminum foil of a purity of99% or above in an acidic solution to thereby enlarge the surface areathereof, and then subjecting the aluminum foil to a anodizing treatmentin an aqueous solution of ammonium borate, ammonium phosphate, ammoniumadipate, etc. to thereby form an anodic oxidation coating layer on thesurface thereof.

The capacitor device (1) thus constructed is then impregnated with anelectrolyte for driving electrolytic capacitors.

The capacitor device (1), which has been thus impregnated with theelectrolyte, is then put into a bottomed outer case (10) made ofaluminum. The outer case (10) is provided at the opening with a sealer(9) and then sealed by drawing. The sealer (9) is made of, for example,an elastic rubber such as butyl rubber having pores through which thelead wires (4) and (5) are to be passed.

Solvent

The first electrolyte contains sulfolane and one or more selected from3-methyl sulfolane and 2,4-dimethyl sulfolane as a solvent. The contentof sulfolane is preferably from 20 to 70% by weight, more preferablyfrom 50 to 70% by weight based on the mixed solvent. The total amount ofone or more selected from 3-methyl sulfolane and 2,4-dimethyl sulfolaneis generally from 30 to 80% by weight, preferably from 30 to 50% byweight based on the mixed solvent.

The second electrolyte contains a mixed solvent containing sulfolane andγ-butyrolactone as a solvent. The content of sulfolane is preferablyfrom 40 to 80% by weight based on the mixed solvent.

The third electrolyte contains a mixed solvent of γ-butyrolactone andtwo or more selected from sulfolane, 3-methyl sulfolane, and2,4-dimethyl sulfolane. The total amount of two or more selected fromsulfolane, 3-methyl sulfolane, and 2,4-dimethyl sulfolaneone ispreferably above 80% by weight based on the mixed solvent.

In each electrolyte, the solvent may further contain other solvents.

Examples of the other solvents which may be mixed therewith includeprotic organic porous solvents such as monohydric alcohols (ethanol,propanol, butanol, pentanol, hexanol, cyclobutanol, cyclopentanol,cyclohexanol, benzyl alcohol, etc.), polyhydric alcohols and oxyalcoholcompounds (ethylene glycol, propylene glycol, glycerol, methylcellosolve, ethyl cellosolve, methoxypropylene glycol,dimethoxypropanol, etc.), aprotic organic porous solvents such as amides(N-methylformamide, N,N-dimethylformamide, N-ethylformamide,N,N-diethylformamide, N-methylacetamide, N,N-dimethylacetamide,N-ethylacetamide, N,N-diethylacetamide, hexamethylphosphoric amide,etc.), lactones (γ-butyrolactone, δ-valerolactone, γ-valerolactone,etc.), cyclic amides (N-methyl-2-pyrrolidone, ethylene carbonate,propylene carbonate, isobutylene carbonate, etc.), nitriles(acetonitrile, etc.), oxides (dimethylsulfoxide, etc.) and2-imidazolidinones (1,3-dialkyl-2-imidazolidinone(1,3-dimethyl-2-imidazolidinone, 1,3-diethyl-2-imidazolidinone,1,3-di(n-propyl)-2-imidazolidinone, etc.),1,3,4-trialkyl-2-imidazolidinone (1.3.4-trimethyl-2-imidazolidinone,etc.).

Solute

Examples of the solute contained in the first electrolyte includeammonium salts, amine salts, quaternary ammonium salts and quaternizedcyclic amidinium salts containing acid conjugated bases as the anioniccomponent which are commonly employed in electrolytes for drivingelectrolytic capacitors. Examples of the amines contained in the aminesalts include primary amines (methylamine, ethylamine, propylamine,butylamine, ethylenediamine, etc.), secondary amines (dimethylamine,diethylamine, dipropylamine, methylethylamine, diphenylamine, etc.) andtertiary amines (trimethylamine, triethylamine, tripropylamine,triphenylamine, 1,8-diazabicyclo(5,4,0)-undecene-7, etc.). Examples ofthe quaternary ammonium contained in the quaternary ammonium saltsinclude tetraalkylammonium (tetramethylammonium, tetraethylammonium,tetrapropylammonium, tetrabutylammonium, methyltriethylammonium,dimethyldiethylammonium, etc.) and pyridium (1-methylpyridium,1-ethylpyridium, 1,3-diethylpyridium, etc.). Examples of the cationscontained in the quaternary salts of the cyclic amidine compoundsinclude cations obtained by quaternizing the following compounds:imidazole monocyclic compounds (imidazole homologs such as1-methylimidazole, 1,2-dimethylimidazole, 1,4-dimethyl-2-ethylimidazoleand 1-phenylimidazole; oxyalkyl imidazole derivatives such as1-methyl-2-oxymethylimidazole and 1-methyl-2-oxyethylimidazole; nitro-and amino-imidazole derivatives such as 1-methyl-4(5)-nitroimidazole and1,2-dimethyl-5(4)-aminoimidazole; benzoimidazole(1-methylbenzoimidazole, 1-methyl-2-benzylbenzoimidazole, etc.);compounds having 2-imidazoline ring (1-methylimidazoline,1,2-dimethylimidazoline, 1,2,4-trimethylimidazoline,1,4-dimethyl-2-ethylimidazoline, 1-methyl-2-phenylimidazoline, etc.);compounds having tetrahydropyrimidine ring(1-methyl-1,4,5,6-tetrahydropyrimidine,1,2-dimethyl-1,4,5,6-tetrahydropyrimidine,1,8-diazabicyclo[5,4,0]undecene-7,1,5-diazabicyclo[4,3,0]nonene-5,etc.).

Examples of the anionic component include conjugated bases of acids suchas carboxylic acids (phthalic acid, isophthalic acid, terephthalic acid,maleic acid, benzoic acid, toluic acid, enanthic acid, malonic acid,etc.), phenols, boric acid, phosphoric acid, carbonic acid and silicicacid.

When a quaternized cyclic amidinium salt is used as the solute, the lowtemperature characteristics can be more improved. Examples of thequaternized cyclic amidinium salt include those wherein a cationiccomponent such as quaternized imidazolinium or quaternized pyrimidiniumis used together with the above-mentioned anionic component. The amountof the quaternized cyclic amidinium salt is preferably from 5 to 40% byweight.

Examples of the quaternized imidazolinium include1,3-dimethylimidazolinium, 1,2,3-trimethylimidazolinium,1,2,3,4-tetramethylimidazolinium, 1-ethyl-3-methylimidazolinium and1-ethyl-2,3-dimethylimidazolinium.

Examples of the quaternized pyrimidinium include1,3-dimethyl-4,5,6-trihydropyrimidinium,1,2,3-trimethyl-4,5,6-trihydropyrimidinium,1,2,3,4-tetramethyl-5,6-dihydropyrimidinium,1-ethyl-3-methyl-4,5,6-trihydropyrimidinium and1-ethyl-2,3-dimethyl-4,5,6-trihydropyrimidinium.

As the solute in the second electrolyte, use may be made of salts havinga conjugated base of an acid as the anionic compound and a quaternizedalkylated imidazoline cation or a quaternized alkylated pyrimidinecation as the cationic component. Examples of the acid for theconjugated base as the anionic component include phthalic acid,isophthalic acid, terephthalic acid, maleic acid, benzoic acid, toluicacid, enanthic acid, malonic acid, phenols, boric acid, phosphoric acid,carbonic acid and silicic acid. Examples of the quaternizedimidazoliniums or quaternized pyrimidiniums serving as the cationiccomponent include those as cited above concerning the first electrolyte.

As the solute in the third electrolyte of the present invention, use ismade of quaternized imidazolinium salts or quaternized pyrimidiniumsalts having quaternized imidazolinium or quaternized pyrimidinium asthe cationic component. Examples of the anionic component includeconjugated bases of acids such as carboxylic acids (phthalic acid,isophthalic acid, terephthalic acid, maleic acid, benzoic acid, toluicacid, enanthic acid, malonic acid, etc.), phenols, boric acid,phosphoric acid, carbonic acid and silicic acid. Examples of thequaternized imidazoliniums or quaternized pyrimidiniums serving as thecationic component include those as cited above concerning the firstelectrolyte.

With respect to the second and third electrolytes, the total amount ofthe quaternized imidazolinium salts and quaternized pyrimidinium saltsis preferably from 5 to 40% by weight.

The first to third electrolytes according to the present invention mayfurther contain boric acid-based compounds such as boric acid, complexcompounds of boric acid with polysaccharides (mannitol, sorbitol, etc.),complex compounds of boric acid with polyhydric alcohols (ethyleneglycol, glycerol, etc.), surfactants, colloidal silica, etc. to therebyimprove the voltage proof.

It is also possible to add various additives thereto in order to lessenleaking current or absorb hydrogen gas. Examples of the additivesinclude aromatic nitro compounds (p-nitrobenzoic acid, p-nitrophenol,etc.), phosphorus compounds (phosphoric acid, phosphorous acid,polyphosphoric acid, acidic phosphate compounds), oxycarboxylic acidcompounds, etc.

Aluminum electrolytic capacitors with the use of the first and thirdelectrolytes as described above are excellent in high temperature lifecharacteristics and, further, in low temperature characteristics. Whenthe above-mentioned first electrolyte contains from 20 to 70% ofsulfolane in the mixed solvent, still preferable low temperaturecharacteristics can be further improved.

When the above-mentioned third electrolyte contains 20% by weight orless of γ-butyrolactone in the mixed solvent, high temperature lifecharacteristics can be further improved.

Aluminum electrolytic capacitors with the use of the second electrolyteof the present invention are excellent in high temperature lifecharacteristics and moisture resistance. Moreover, these electrolyticcapacitors have a low dielectric loss and good low temperaturecharacteristics and suffer from little liquid leakage.

When the second electrolyte as described above contains 60% or less ofγ-butyrolactone, the life characteristics can be more improved. When thecontent thereof is larger than 20%, the dielectric loss and the lowtemperature characteristics are improved. That is to say, excellent hightemperature life characteristics, a low dielectric loss and good lowtemperature characteristics can be achieved, when the content ofγ-butyrolactone is from 20 to 60%.

In the conventional electrolytes containing quaternized cyclic amidiniumsalts such as quaternized imidazolinium salts or quaternizedpyrimidinium salts as the solute, γ-butyrolactone is employed as thesolvent. However, these electrolytes suffer from a problem that theelectrolytes would leak between the sealer (9) and the round bar (6) ofthe lead wire. In contrast thereto, no liquid leakage is observed in thecase of the electrolytes of the present invention. The reason thereforis seemingly as follows.

An electrolyte having a quaternized cyclic amidinium salt dissolvedtherein would leak in the following manner. Namely, in a conventionalelectrolytic capacitor, the spontaneous immersion potential of thecathode lead wire (5) is noble as compared with the spontaneousimmersion potential of the cathodic electrode foil (3). When a directcurrent is loaded, therefore, more cathode current passes through thecathode lead wire than through the cathodic electrode foil. When no loadis applied, a local cell is formed by the cathode lead wire and thecathode foil and thus a cathode current passes through the cathode leadwire. That is to say, a cathode current passes through the cathode leadwire both in the loaded and unloaded state. As a result, there arises areduction reaction of the dissolved oxygen or hydrogen ion in the sideof the cathode lead wire and hydroxyl ion is formed at the interfacebetween the round bar (6) of the cathode lead wire and the electrolyteand at the interface between the connecting member (7) of the cathodelead wire and the electrolyte.

Then, the hydroxyl ion thus formed by the reduction reaction of thedissolved oxygen or hydrogen ion reacts with a quaternized cyclicamidinium. The quaternized cyclic amidinium undergoes ring-opening togive a secondary amine. Because of being highly volatile and lesshygroscopic, it is expected that this secondary amine formed in thespace between the round bar and the sealer would quickly volatilizewithout causing liquid leakage.

When hydroxyl ion is formed, however, γ-butyrolactone employed as thesolvent also reacts therewith to give γ-hydroxybutyric acid. That is tosay, there are the above-mentioned secondary amine together with thisγ-hydroxybutyric acid. Due to the pH value-lowering effect ofγ-hydroxybutyric acid, the secondary amine formed by the ring-opening ofthe quaternized cyclic amidinium undergoes ring-closure to give thequaternized cyclic amidinium salt again. Since the quaternized cyclicamidinium salt thus formed is not volatile but highly hygroscopic, therearises liquid leakage from the space between the round bar of thecathode lead wire and the sealer due to the hygroscopicity of thequaternized cyclic amidinium salt. It is estimated that the liquidleakage proceeds in accordance with the above mechanism based on theanalytical data that the leaking liquid is mainly composed of water asthe major component and the quaternized cyclic amidinium salt.

In the first electrolyte of the present invention, in contrast thereto,use is made as the solvent of a mixed solvent of sulfolane and at leastone member selected from 3-methyl sulfolane and 2,4-dimethyl sulfolane.Since none of 3-methyl sulfolane, 2,4-dimethyl sulfolane and sulfolanereacts with hydroxyl ion, no compound lowering pH value, asγ-hydroxybutyric acid does, is formed in this case. Thus, thequaternized cyclic amidinium salt is not formed again and the generatedsecondary amine volatilizes without causing any liquid leakage.

In the second electrolyte of the present invention, use is made as thesolvent of a mixed solvent of sulfolane with γ-butyrolactone. In thethird electrolyte of the present invention, use is made as the solventof a mixed solvent of γ-butyrolactone and at least two members selectedfrom among sulfolane, 3-methyl sulfolane and 2,4-dimethyl sulfolane. Inthese cases, the liquid leakage is prevented too.

Since none of sulfolane, 3-methyl sulfolane and 2,4-dimethyl sulfolanereacts with hydroxyl ion, no compound lowering pH value, as theabove-mentioned one does, is formed in these cases. Even if a substancelowering pH value such as γ-hydroxybutyric acid is formed fromγ-butyrolactone, the effect is weak. Therefore, the amount of thequaternized cyclic amidinium salt regenerated by ring-closure of thesecondary amine (formed by the ring-opening of the quaternized cyclicamidinium) is small, and the formed secondary amine volatilizes, wherebyliquid leakage is prevented.

When a reverse voltage is applied, a cathode current flows in the anodicside in general. Since the polarization resistance of the anodic foil isextremely higher than that of the cathodic foil, the major part of thecathode current in the anodic side passes through the anode lead wire.In conventional electrolytic capacitors, therefore, it is sometimesobserved that liquid leakage from the anode lead wire arises from theearly stage in a reverse voltage test. On the other hand, when a reversevoltage is applied to the electrolytic capacitor of the presentinvention, there arises no liquid leakage too. The liquid leakage isinhibited in the reverse voltage test too, seemingly because of theeffect of the electrolyte of the present invention similar to the oneobserved in the cathode side as described above. Namely, the presentinvention achieves an extremely remarkable effect of preventing liquidleakage.

According to the constitution of the present invention as describedabove, hydroxyl ion generated around the round bar of the cathode leadwire reacts with the quaternized cyclic amidinium and thus disappears.Therefore, the amount of the quaternized cyclic amidinium generatedagain is small and the secondary amine thus formed volatilizes. Thus,the liquid leakage is prevented in this case.

When a conventional electrolytic capacitor is allowed to stand in anunloaded state and the cathode lead wire (4) comes into contact with theanode lead wire (5), the anode lead wire forms a local cell togetherwith the cathodic electrode foil (3) As a result, there arises areduction reaction of the dissolved oxygen or hydrogen ion in the anodelead wire side to thereby give hydroxyl ion. Thus, liquid leakage arisestoo, as observed in the cathode lead wire part. According to theconstitution of the present invention, the electrolyte of the presentinvention can also prevent this liquid leakage based on the similarmechanism as the one observed in the cathode lead wire part.

Therefore, it is considered that liquid leakage can be prevented in thepresent invention for the above-mentioned reasons.

As the cathodic electrode foil (3), use can be made of a cathodicelectrode foil having formed thereon a coating made of a metal nitrideselected from the group consisting of titanium nitride, zirconiumnitride, tantalum nitride and niobium nitride or a metal selected fromthe group consisting of titanium, zirconium, tantalum and niobium byknown methods such as metallizing, plating or application. The coatinggenerally has a thickness of 0.01 to 0.5 μm. The cathodic electrode foilmay be coated with the metal nitride or metal either on the wholesurface or a part of the same, for example, on only one face of thefoil. Thus, the spontaneous immersion potential of the cathode foilbecomes noble as compared with that of the cathode lead wire and thecathode polarization resistance is lowered. When an over-voltage isapplied, the cathode current in the cathode lead wire becomes minor andthe formation of hydroxyl ion in the cathode lead wire side isinhibited, which is further appropriate in preventing liquid leakage.

It is possible to form an aluminum oxide layer by anodic oxidation withthe use of an aqueous solution of ammonium borate, ammonium phosphate orammonium adipate or to form an insulating layer such as a ceramiccoating layer composed of Al₂O₃, SiO₂, ZrO₂, etc. on the surface of atleast the round bar (6) of the lead wires (4) and (5). The thickness ofthe insulating layer is generally from 2 to 25 μm. In an unloaded state,the area constituting the local cell of the cathode lead wire and thecathode foil is reduced. In a loaded state, the cathode current passingthrough the cathode lead wire is reduced. In both of these cases,therefore, the formation of hydroxyl ion in the cathode lead wire sideis inhibited and the effect of preventing liquid leakage is furtherimproved.

EXAMPLES

Next, Examples of the first electrolyte of the present invention will begiven.

Tables 1 and 2 show the composition and specific resistance at 30° C.and −40° C. of the electrolyte for electrolytic capacitors of eachExample.

TABLE 1 Composition of (wt. %) Specific Resistance (Ω-cm) 3-MSL SL EGEDMIP TMAP AAd 30° C. −40° C. Example 1a 65 10(13) 0 25 0 0 323 28kExample 2a 60 15(20) 0 25 0 0 320 20k Example 3a 50 25(33) 0 25 0 0 31615k Example 4a 37.5 37.5(50) 0 25 0 0 312 13k Example 5a 22.5 52.5(70) 025 0 0 296 20k Example 6a 15 60(80) 0 25 0 0 292 28k Example 7a 37.537.5(50) 0 0 25 0 420 20k Comp. Example 1a 0 75 0 25 0 0 285 solidifiedComp. Example 2a 75 0 0 25 0 0 325 solidified Prior Example a 0 0 87 0 013 320 solidified 3-MSL: 3-methyl sulfolane. SL: sulfolane. EG: ethyleneglycol. EDMIP: 1-ethyl-2,3-dimethylimidazolinium phthalate. TMAP:tetramethylammonium phthalate. AAd: ammonium adipate. Values inparentheses given in SL: content (wt. %) of sulfolane in mixed solvent.

TABLE 2 Composition of Electrolyte (wt. %) Specific Resistance (Ω-cm)2,4-DMSL SL EG EDMIP TMAP AAd 30° C. −40° C. Example 9a 65 10(13) 0 25 00 339 23k Example 10a 60 15(20) 0 25 0 0 336 18k Example 11a 50 25(33) 025 0 0 330 11k Example 12a 37.5 37.5(50) 0 25 0 0 320 10k Example 13a22.5 52.5(70) 0 25 0 0 306 16k Example 14a 15 60(80) 0 25 0 0 301 22kExample 15a 37.5 37.5(50) 0 0 25 0 460 19k Comp. Example 1a 0 75 0 25 00 285 solidified Comp. Example 3a 75 0 0 25 0 0 344 solidified PriorExample a 0 0 87 0 0 13 320 solidified *2,4-DMSL: 2,4-dimethyl sulfolaneValues in parentheses given in SL: content (wt. %) of sulfonate in mixedsolvent.

In Example 8a, 1 part of p-nitrobenzoic acid and 0.3 parts of phosphoricacid were added to 100 parts of the electrolyte of Example 4a. Theelectrolyte thus obtained showed specific resistance of 320 Ω-cm at 30°C. and 14 kΩ-cm at −40° C. In Example 16a, 1 part of p-nitrobenzoic acidand 0.3 parts of phosphoric acid were added to 100 parts of theelectrolyte of Example 12a. The electrolyte thus obtained showedspecific resistance of 328 Ω-cm at 30° C. and 11 kΩ-cm at −40° C.

As Tables 1 and 2 clearly show, the electrolytes of Examples 1a to 15aaccording to the present invention were comparable or superior inspecific resistance at 30° C. and −40° C. to the electrolytes ofComparative Examples 1a to 3a with the use of sulfolane, 3-methylsulfolane or 2,4-dimethyl sulfolane alone and the one of Prior Examplewith the use of ethylene glycol and ammonium adipate. In particular, theelectrolytes of Examples 2a to 5a, 8a, 10a to 13a and 16a showing asulfolane content of from 20 to 70% sustained low specific resistanceeven at −40° C., which indicates that these electrolytes are usable at−40° C. In contrast, the electrolytes of Comparative Examples la to 3aand Prior Example were solidified at −40° C., which indicates that theycannot be employed at −40° C. The electrolyte of Prior Example a showedspecific resistance of 9 kΩ-cm at −25° C.

The electrolytes of Examples 4a and 12a wherein1-ethyl-2,3-dimethylimidazolinium phthalate was employed as the soluteshowed lower specific resistances both at 30° C. and −40° C. than thoseof the electrolytes of Examples 7a and 15a wherein tetramethylammoniumphthalate was employed as the solute.

To evaluate the high temperature life characteristics, aluminumelectrolytic capacitors were constructed by using the electrolytes ofExamples 2a, 5a, 10a and 13a and that of Prior Example. The rated valuesof the aluminum electrolytic capacitors employed herein were 16V-47 μFand the case size thereof was 6.3 mm (diameter) ×5 mm. The rated voltagewas applied onto 25 samples of each electrolytic capacitor at 125° C.and the change in electrostatic capacity (ΔC) and the tangent of lossangle (tanδ) were measured after 2,000 hours and 4,000 hours. Table 3summarizes the results.

TABLE 3 Initial Charac- teristics 2,000 hrs 4,000 hrs Cap tanδ ΔC tanδΔC tanδ Example 2a 46.1 0.13 −6.0 0.18 −12.1 0.30 Example 5a 46.2 0.13−5.2 0.16 −9.0 0.26 Example 10a 46.2 0.14 −6.1 0.19 −11.5 0.31 Example13a 46.5 0.13 −5.4 0.17 −8.9 0.28 Prior Example a 46.9 0.12 −22 0.60 —— * Cap (μF), ΔC (%).

As Table 3 clearly shows, the electrolytic capacitors of Examples 2a,5a, 10a and 13a were superior in high temperature lifecharacteristics-and sustained low initial tanΔ compared with theelectrolytic capacitor of Prior Example a. These data ensure that theelectrolytic capacitors of the present invention are usable at 125° C.for 4,000 hours.

To evaluate the liquid leakage characteristics, electrolytic capacitorswith the use of the electrolytes of Examples 4a and 11a and anelectrolytic capacitor (Comparative Example 4a) with the use of anelectrolyte comprising 75% by weight of γ-butyrolactone and 25% byweight of 1-ethyl-2,3-dimethylimidazolinium phthalate were constructed.The rated voltage was applied onto 25 samples of each electrolyticcapacitor at 85° C. under 85% RH and the occurrence of liquid leakagewas monitored with the naked eye after 500, 1,000 and 2,000 hours. Table4 summarizes the results. Moreover, a reverse voltage of −1.5 V wasapplied onto 25 samples of each electrolytic capacitor at 85° C. under85% RH and the occurrence of liquid leakage was monitored with the nakedeye after 250, 500 and 1,000 hours. Table 5 summarizes the results.

TABLE 4 500 hrs 1,000 hrs 2,000 hrs Example 4a 0/25 0/25 0/25 Example11a 0/25 0/25 0/25 Comp. Example 4a 0/25 10/25  25/25 

TABLE 5 250 hrs 500 hrs 1,000 hrs Example 4a 0/25 0/25 0/25 Example 11a0/25 0/25 0/25 Comp. Example 4a 5/25 20/25  25/25 

As Table 4 clearly shows, liquid leakage arose after 1,000 hours, in theelectrolytic capacitor of Comparative Example 4a, while the electrolyticcapacitors with the use of the electrolytes of Examples 4a and 11aaccording to the present invention showed no liquid leakage even after2,000 hours, thus achieving good results. As table 5 shows, furthermore,the electrolytic capacitor of Comparative Example 4a suffered fromliquid leakage after 250 hours in the reverse voltage test, while thoseof the present invention were free from any liquid leakage even after1,000 hours, thus achieving considerably strong effects of preventingliquid leakage. These facts indicate that the electrolytes of thepresent invention are highly effective in preventing liquid leakage.

As described above, the first electrolyte for electrolytic capacitorscontains a mixed solvent of at least one member selected from 3-methylsulfolane and 2,4-dimethyl sulfolane with sulfolane as the solvent.Electrolytic capacitors with the use of this electrolyte have excellenthigh temperature life characteristics and good low temperaturecharacteristics. By controlling the content of the sulfolane to 20 to70% by weight based on the whole solvent, further improved lowtemperature characteristics can be obtained. By using a quaternizedcyclic amidinium salt such as a quaternized imidazolinium salt or aquaternized pyrimidinium salt as the solute, the low temperaturecharacteristics can be further improved. When a quaternized cyclicamidinium salt is used in the electrolyte, no liquid leakage arises.

Next, the electrolytic capacitor containing the second electrolyte ofthe present invention will be illustrated by reference to Examples. AsFIG. 1 shows, a capacitor device is constructed by winding, via aseparator (11), an anodic electrode foil (2) and a cathodic electrodefoil (3). As FIG. 2 shows, the anodic electrode foil (2) and thecathodic electrode foil (3) are connected respectively to ananode-leading wire (4) and a cathode-leading wire (5).

These lead wires (4) and (5) are each composed of a connecting member(7) being in contact with the electrode foil, a round bar (6) havingbeen molded integrally with the connecting member (7), and an outerconnecting member (8) having been fixed at the tip of the round bar (6).The connecting member (7) and the round bar (6) are each made ofaluminum having a purity of 99% while the outer connecting member (8) ismade of a solder-plated, copper-plated steel wire. These lead wires (4)and (5) are electrically connected respectively to the electrode foils(2) and (3) at the connecting member (7) by stitching, ultrasonicwelding, etc.

As the anodic electrode foil (2), use is made of one prepared bychemically or electrochemically etching an aluminum foil of 99.9% inpurity in an acidic solution to thereby enlarge the area and thensubjecting the foil to a anodizing treatment in an aqueous ammoniumadipate solution to form an anodic oxidation coating layer thereon. Asthe cathodic electrode foil (3), use is made of one prepared by etchingan aluminum foil of 99.7% in purity.

The capacitor device (1) thus constructed is then impregnated withelectrolytes for driving aluminum electrolytic capacitors. Table 6 showsthe composition and electric conductance at 30° C. and −40° C. of eachelectrolyte.

TABLE 6 Composition of Electrolyte (wt. %) Electric Conductance (mS/cm)Sulfolane GBL EDMIP TMAP TEPA 30° C. −40° C. Example 1b 67.5 7.5(10) 254.2  0.05 Example 2b 60 15(10) 25 4.9 0.1 Example 3b 50 25(33) 25 6.50.2 Example 4b 37.5 37.5(50) 25 8.0 0.5 Example 5b 30 45(60) 25 8.6 0.7Example 6b 22.5 52.5(70) 25 10.0 0.8 Comp. Example 1b 75 25 3.3solidified Comp. Example 2b 60 15(20) 25 3.1 solidified Comp. Example 3b50 25(33) 25 4.5 0.1 Comp. Example 4b 37.5 37.5(53) 25 6.1 0.3 Comp.Example 5b 50 25(33) 25 2.0  0.05 *GBL: γ-butyrolactone. EDMIP:1-ethyl-2,3-dimethylimidazolinium phthalate. TMAP: tetramethylammoniumphthalate. TEAP: triethylammonium phthalate. Values in parentheses givenin SL: content (wt. %) of sultonate in mixed solvent.

As Table 6 clearly shows, the electrolytes of Examples 1b to 6baccording to the present invention were superior in electric conductanceat 30° C. and −40° C. to the electrolytes of Comparative Examples 2b to5b with the use of tetramethylammonium phthalate or triethylammoniumphthalate as the solute. The electrolytes of Examples 2b to 6b with aγ-butyrolactone content of 20% or above sustained high electricconductance even at −40° C. In contrast, the electrolytes of ComparativeExample 1b with the use of sulfolane alone as the solvent solidified at−40° C.

To evaluate the high temperature life characteristics, the capacitordevices (1) were impregnated with the electrolytes of Examples 2b and 6band that of Prior Example 1b comprising 75% of γ-butyrolactone and 25%1-ethyl-2,3-dimethylimidazolinium phthalate. Next, these capacitordevices each were put into an outer case (10) which was a bottomed tubemade of aluminum. Then, the opening of the outer case (10) was sealedwith a sealer (9) by drawing.

The rated values of the aluminum electrolytic capacitors employed thusconstructed were 16V-47 μF and the case size thereof was 6.3 mm(diameter) ×5 mm. The rated voltage was applied onto 25 samples of eachof the electrolytic capacitors of Examples 2b and 6b and Prior Example1b at 125° C. and the change in electrostatic capacity (ΔC) and thetangent of loss angle (tanδ) were measured after 1,000 hours and 2,000hours. Table 7 summarizes the results.

TABLE 7 Initial Characteristics 1,000 hrs 2,000 brs Cap tanδ ΔC tanδ ΔCtanδ Example 2b 46.0 0.09 −8.3 0.11 −13.3 0.18 Example 6b 46.3 0.08−14.5 0.18 −26.6 0.35 Prior Example 1b 46.5 0.07 −43.3 1.05 — — *Cap(μF), ΔC (%), LC(μA).

As Table 7 clearly shows, the electrolytic capacitors of Examples 2b and6b were superior in high temperature life characteristics and sustainedlow initial tanΔ compared with the electrolytic capacitor of PriorExample 1b with the use of γ-butyrolactone alone as the solvent. Inparticular, the electrolytic capacitor of Example 2b the γ-butyrolactonecontent of which in the solvent fell within the range of from 20 to 60%sustained the characteristics at 125° C. for 2,000 hours. In contrastthereto, the electrolytic capacitor of Example 6b suffered fromdeterioration in its characteristics after 2,000 hours.

To evaluate the moisture resistance, electrolytes were prepared byadding 6% of water content to the electrolytes of Example 2b and PriorExample 1b to give electrolytes of Example 7b and Comparative Example 6brespectively. Then electrolytic capacitors were constructed by the samemethod as the one described above. 25 samples of each of theseelectrolytic capacitors were allowed to stand at 125° C. and the changein electrostatic capacity (ΔC), the tangent of loss angle (tanδ) andleaking current (LC) were measured after 1,000 hours and 2,000 hours.Table 8 summarizes the results.

TABLE 8 Initial Characteristics 1,000 hrs 2,000 hrs Cap tan LC ΔC tan LCΔC tan LC Example 7b 46.3 0.08 0.3 −12.0 0.15 2.8 −21.4 0.26 4.3 Comp.Ex. 46.6 0.06 0.3 −83.0 3.8 36.3 — — — 6b *Cap (μF), ΔC (%), LC(μA).

As Table 8 clearly shows, the electrolyte of Example 7b obtained byadding 6% of water content to the electrolyte of the present inventionwas superior in all of the items examined (i.e., change in electrostaticcapacity, tangent of loss angle and leaking current) to the electrolyteof Comparative Example 6b prepared by adding 6% of water content to theprior electrolyte, which indicates that the electrolytic capacitor ofthe present invention has an improved moisture resistance.

To evaluate the liquid leakage characteristics, an electrolyticcapacitor was constructed in the same manner as in Example 2b except forcoating the whole surface of the cathodic electrode foil (3) withtitanium nitride by vacuum evaporation method(Example 8b).

Furthermore, another electrolytic capacitor was constructed in the samemanner as in Example 8b except for using an cathodic electrode foil inwhich an aluminum oxide layer had been formed at least on the surface ofthe round bar (6) of the lead wires (4) and (5) by anodic oxidation withthe use of an aqueous solution of ammonium phosphate (Example 9b).

Onto 25 samples of each of the electrolytic capacitors of theabove-mentioned Examples 2b, 8b and 9b and another electrolyticcapacitor (Prior Example 2b) with the use of an electrolyte comprising75% of γ-butyrolactone and 25% of tetramethylammonium phthalate andanother electrolytic capacitor (Prior Example 3b) with the use of anelectrolyte comprising 75% of γ-butyrolactone and 25% of1-ethyl-2,3-dimethylimidazolinium phthalate, the rated voltage wasapplied at 125° C. and the occurrence of liquid leakage was monitoredwith the naked eye after 1,500, 3,000 and 5,000 hours. Table 9summarizes the results.

TABLE 9 1,500 hrs 3,000 hrs 5,000 hrs Example 2b 0/25 15/25  25/25 Example 8b 0/25 0/25 2/25 Example 9b 0/25 0/25 0/25 Prior Example 2b25/25  — — Prior Example 3b 25/25  — —

As Table 9 clearly shows, the electrolytic capacitor of Example 2b withthe use of the electrolyte according to the present invention showed noliquid leakage after 1,500 hours and, therefore, was superior to theelectrolytic capacitors of Prior Examples 2b and 3b. The electrolyticcapacitor of Example 2b achieved good results at 125° C. too. Theelectrolytic capacitor of Example 8b, wherein the electrolyte of Example2b was employed and the whole surface of the cathodic electrode foil hadbeen coated with titanium nitride, showed less liquid leakage.Furthermore, the electrolytic capacitor of Example 9b, wherein analuminum oxide layer had been formed on the surface of the round bar ofthe lead wire in the electrolytic capacitor of Example 8b, showed lessliquid leakage.

Moreover, a reverse voltage of −1.5 V was applied onto 25 samples ofeach of the electrolytic capacitors with the use of the electrolytes ofExamples 2b, 8b and 9b and Prior Examples 2b and 3b at 85° C. under 85%RH and the occurrence of liquid leakage was monitored with the naked eyeafter 250, 500 and 1,000 hours. Table 10 summarizes the results.

TABLE 10 250 hrs 500 hrs 1,000 hrs Example 2b 0/25 0/25 0/25 Example 8b0/25 0/25 0/25 Example 9b 0/25 0/25 0/25 Prior Example 2b 15/25  25/25 — Prior Example 3b 5/25 20/25  25/25 

As Table 10 clearly shows, liquid leakage arose in the electrolyticcapacitors of Prior Examples 2b and 3b after 250 hours and all samplessuffered from liquid leakage after 500 and 1,000 hours in the reversevoltage test. On the other hand, the electrolytic capacitors accordingto the present invention showed no liquid leakage even after 1,000hours, thus achieving good results. As described above, the electrolyticcapacitors of the present invention can establish excellent effect ofpreventing liquid leakage.

As described above, the present invention provides an aluminumelectrolytic capacitor constructed by winding, via a separator, ananodic electrode foil provided with an anode leading means and acathodic electrode foil provided with a cathode leading means made ofaluminum to form a capacitor device, then impregnating the capacitordevice with an electrolyte wherein a quaternized imidazolinium salt or aquaternized pyrimidinium salt is dissolved as the solute in a mixedsolvent containing sulfolane and γ-butyrolactone, and then putting thethus impregnated capacitor device in an outer case.

This electrolytic capacitor is excellent in high temperature lifecharacteristics and moisture resistance. It also excellent in dielectricloss and low temperature characteristics and suffers from little liquidleakage. By controlling the content of γ-butyrolactone in the mixedsolvent in the above-mentioned electrolyte to 20 to 60% by weight basedon the whole solvent, it is possible to obtain further improved hightemperature life characteristics, lower dielectric loss and better lowtemperature characteristics. The liquid leakage can be further relievedby using an aluminum foil provided with a coating made of a metalnitride selected from the group consisting of titanium nitride,zirconium nitride, tantalum nitride and niobium nitride or a metalselected from the group consisting of titanium, zirconium, tantalum andniobium formed on the whole surface or a part of the same. The liquidleakage characteristics can be further improved by further forming analuminum oxide layer on the whole surface or a part of the same of thecathode leading means by anodic oxidation.

Next, the third electrolyte of the present invention will be illustratedby reference to Examples. Tables 11 to 13 show the composition andspecific resistance at 30° C. and −40° C. of the electrolyte forelectrolytic capacitors of each Example according to the presentinvention.

TABLE 11 Composition of Electrolyte (wt. %) Specific Resistance (Ω-cm)SL 3-MSL GBL EDMIP TMAP 30° C. −40° C. Example 1c 63 7 5(7) 25 0 260 24kExample 2c 7 63 5(7) 25 0 276 18k Example 3c 58.5 6.5 10(14) 25 0 23215k Example 4c 32.5 32.5 10(14) 25 0 242 9.5k Example 5c 6.5 58.5 10(14)25 0 250 11k Example 6c 54 6 15(20) 25 0 207 10k Example 7c 6 54 15(20)25 0 223 8.8k Example 8c 28.5 28.5 18(24) 25 0 213 6.9k Comp. Example 1c32.5 32.5 10(14) 0 25 295 13k Comp. Example 4c 75 0 0 25 0 285solidified Comp. Example 5c 0 75 0 25 0 325 solidified *SL: sulfolane.3-MSL: 3-methyl sulfolane. GBL: γ-butyrolactone. EDMIP:1-ethyl-2,3-dimethylimidazolinium phthalate. TMAP: tetramethylammoniumphthalate. Values in parentheses given in GBL: content (wt. %) ofγ-butyrolactone in mixed solvent.

TABLE 12 Composition of Electrolyte (wt. %) Specific Resistance (Ω-cm)SL 2,4-DMSL GBL EDMIP TMAP 30° C. −40° C. Example 10c 63 7 5(7) 25 0 26822k Example 11c 7 63 5(7) 25 0 290 17k Example 12c 58.5 6.5 10(14) 25 0255 14k Example 13c 32.5 32.5 10(14) 25 0 262 9.0k Example 14c 6.5 58.510(14) 25 0 269 11k Example 15c 54 6 15(20) 25 0 219 9.5k Example 16c 654 15(20) 25 0 238 8.5k Example 17c 28.5 28.5 18(24) 25 0 225 6.7k Comp.Example 2c 32.5 32.5 10(14) 0 25 318 13k Comp. Example 6c 0 75 0 25 0344 solidified *2,4-DMSL: 2,4-dimethyl sulfolane.

TABLE 13 Composition of Electrolyte (wt. %) Specific Resistance (Ω-cm)3-MSL 2,4-DMSL GBL EDMIP TMAP 30° C. −40° C. Example 19c 63 7 5(7) 25 0280 21k Example 20c 7 63 5(7) 25 0 303 16k Example 21c 58.5 6.5 10(14)25 0 270 14k Example 22c 32.5 32.5 10(14) 25 0 278 8.3k Example 23c 6.558.5 10(14) 25 0 285 12k Example 24c 54 6 15(20) 25 0 229 9.5k Example25c 6 54 15(20) 25 0 248 8.6k Example 26c 28.5 28.5 18(24) 25 0 234 7.0kComp. Example 32.5 32.5 10(14) 0 25 348 13k 3c

In Example 9c, 1 part of p-nitrobenzoic acid and 0.3 parts of phosphoricacid were added to 100 parts of the electrolyte of Example 4c. Theelectrolyte thus obtained showed specific resistance of 246 Ω-cm at 30°C. and 9.6kΩ-cm at −40° C.

In Example 18c, part of p-nitrobenzoic acid and 0.3 parts of phosphoricacid were added to 100 parts of the electrolyte of Example 13c. Theelectrolyte thus obtained showed specific resistance of 268 Ω-cm at 30°C. and 9.2kΩ-cm at −40° C.

In Example 27c, part of p-nitrobenzoic acid and 0.3 parts of phosphoricacid were added to 100 parts of the electrolyte of Example 22c. Theelectrolyte thus obtained showed specific resistance of 282 Ω-cm at 30°C. and 8.5kΩ-cm at −40° C.

In Prior Example c, 13% by weight of ammonium adipate was dissolved in87% by weight of ethylene glycol. The electrolyte thus obtained showedspecific resistance of 320 Ω-cm at 30° C. and was solidified at −40° C.

As Tables 11 to 13 clearly show, the electrolytes of Examples 1c to 30caccording to the present invention were excellent in specific resistanceat 30° C. and −40° C. In particular, they maintained good specificresistance even at −40° C., which indicates that they are usable at −40°C. In contrast, the electrolyte of Prior Example c could not be employedat −40° C. because of solidification. The electrolyte of Prior Example cshowed specific resistance of 9kΩ-cm at −25° C.

The electrolytes of Examples 4c, 13c and 22c with the use of1-ethyl-2,3-dimethylimidazolinium phthalate as the solute sustained lowspecific resistance compared with those of Comparative Examples 1c, 2cand 3c with the use of tetramethylammonium phthalate both at 30° C. and−40° C. The electrolytes of Comparative Examples 4c, 5c and 6c with theuse of sulfolane, 3-methyl sulfolane or 2,4-dimethyl sulfolane alonewere solidified even at −25° C., which indicates that these solvents areunusable in electrolytes for electrolytic capacitors.

To evaluate the high temperature life characteristics, aluminumelectrolytic capacitors were constructed by using the electrolytes ofExamples 2c, 4c, 8c, 11c, 13c, 17c, 20c, 22c and 26c and that of PriorExample c. The rated values of the aluminum electrolytic capacitorsemployed thus constructed were 16V-47μF and the case size thereof was6.3 mm (diameter)×5 mm. The rated voltage was applied onto 25 samples ofeach of the electrolytic capacitors at 125° C. and the change inelectrostatic capacity (ΔC) and the tangent of loss angle (tanδ) weremeasured after 2,000 hours and 4,000 hours. Table 14 summarizes theresults.

TABLE 14 Initial Chara- cteristics 2,000 hrs 4,000 hrs Cap tanδ ΔC tanδΔC tanδ Example 2c 46.2 0.12 −6.0 0.18 −12.1 0.27 Example 4c 46.3 0.11−8.1 0.19 −18.0 0.33 Example 8c 46.3 0.10 −14.1 0.20 −26.5 0.40 Example11c 46.1 0.13 −6.4 0.19 −12.8 0.29 Example 13c 46.3 0.12 −8.6 0.22 −19.00.35 Example 17c 46.2 0.10 −15.2 0.24 −30.1 0.45 Example 20c 46.3 0.13−7.1 0.19 −14.2 0.29 Example 22c 46.2 0.12 −9.2 0.24 −19.9 0.39 Example26c 46.4 0.11 −15.8 0.26 −31.5 0.46 Prior Example c 46.9 0.12 −22 0.60 −− *Cap (μF), ΔC (%), LC(μA).

As Table 14 clearly shows, the electrolytic capacitors of Examples weresuperior in high temperature life characteristics and sustained lowinitial tanδ compared with the electrolytic capacitor of Prior Examplec. Thus, the electrolytic capacitors according to the present inventionare usable at 125° C. for 4,000 hours. In particular, the electrolyticcapacitors of Examples 2c, 4c, 11c, 13c, 20c and 22c containing 20% byweight or less of γ-butyrolactone sustained excellent characteristicsafter 4,000 hours.

To evaluate the liquid leakage characteristics, electrolytic capacitorswere constructed by using the electrolytes of Examples 4c, 13c and 22cand Comparative Examples 1c to 3c and another comparative electrolyte(Comparative Example 7c) comprising 75% by weight of γ-butyrolactone and25% by weight of 1-ethyl-2,3-dimethylimidazolinium phthalate. Onto 25samples of each of these electrolytic capacitors, the rated voltage wasapplied at 85° C. under 85% RH and the occurrence of liquid leakage wasmonitored with the naked eye after 500, 1,000 and 2,000 hours. Table 15shows the results. Moreover, a reverse voltage of −1.5 V was appliedonto 25 samples of each of the electrolytic capacitors of Examples 4c,13c and 22c, and Comparative Example 7c at 85° C. under 85% RH and theoccurrence of liquid leakage was monitored with the naked eye after 250,500 and 1,000 hours. Table 16 summarizes the results.

TABLE 15 500 hrs 1,000 hrs 2,000 hrs Example 4c 0/25 0/25 0/25 Example13c 0/25 0/25 0/25 Example 22c 0/25 0/25 0/25 Comp. Example 1c 8/2525/25  — Comp. Example 2c 10/25  25/25  — Comp. Example 3c 11/25  25/25 — Comp. Example 7c 0/25 10/25  25/25 

TABLE 15 500 hrs 1,000 hrs 2,000 hrs Example 4c 0/25 0/25 0/25 Example13c 0/25 0/25 0/25 Example 22c 0/25 0/25 0/25 Comp. Example 1c 8/2525/25  — Comp. Example 2c 10/25  25/25  — Comp. Example 3c 11/25  25/25 — Comp. Example 7c 0/25 10/25  25/25 

As Table 15 clearly shows, the electrolytic capacitors of ComparativeExamples 1c to 3c with the use of quaternary ammonium salts sufferedfrom liquid leakage after 500 hours, while the one of ComparativeExample 7c with the use of γ-butyrolactone as the solvent suffered fromliquid leakage after 1,000 hours. In contrast, the electrolyticcapacitors with the use of the electrolytes of the present invention ofExamples 4c, 13c and 22c showed no liquid leakage even after 2,000hours, thus achieving favorable results. In the reverse voltage test, asTable 16 clearly shows, the electrolytic capacitor of ComparativeExample 7c suffered from liquid leakage after 250 hours, while those ofthe present invention showed no liquid leakage even after 1,000 hours,thus showing highly remarkable effect of preventing liquid leakage.Thus, it can be understood that the electrolytes of the presentinvention are effective in preventing liquid leakage.

As described above, the third electrolyte for electrolytic capacitorsaccording to the present invention is one wherein a mixed solvent of atleast two members selected from sulfolane, 3-methyl sulfolane and2,4-dimethyl sulfolane with γ-butyrolactone is used as the solvent and aquaternized imidazolinium salt or a quaternized pyrimidinium salt isused as the solute.

The electrolytic capacitor with the use of this electrolyte is excellentin high temperature life characteristics and low temperaturecharacteristics. In the above-mentioned electrolyte, further improvedhigh temperature life characteristics can be obtained by controlling thecontent of γ-butyrolactone in the mixed solvent to 20% by weight or lessbased on the whole solvent. Moreover, the electrolytic capacitor of thepresent invention is free from liquid leakage.

While the invention has been described in detail and with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof.

What is claimed is:
 1. An electrolyte for electrolytic capacitors whichcomprises a mixed solvent of sulfolane and at least one member selectedfrom the group consisting of 3-methyl sulfolane and 2,4-dimethylsulfolane as a solvent and a quaternized cyclic ainidiniumn salt as asolute.
 2. The electrolyte for electrolytic capacitors of claim 1,wherein the content of sulfolane is from 20 to 70% by weight based onthe mixed solvent.
 3. An electrolytic capacitor, which comprises anelectrolyte comprising a mixed solvent of sulfolane and at least onemember selected from the group consisting of 3-methyl sulfolane and2,4-dimethyl sulfolane as a solvent and a quaternized cyclic amidiniumsalt as a solute.
 4. The electrolytic capacitor of claim 3, wherein thecontent of sulfolane is from 20 to 70% by weight based on the mixedsolvent.